In the course of completing an oil and/or gas well, it is common practice to run a string of casing into the well bore and then to run the production tubing inside the casing. At the site of the producing formation, the casing is perforated across one or more production zones to allow production fluids to enter the casing bore. After the well is completed and placed in production, formation sand from unconsolidated formations may be swept into the flow path along with formation fluid, which erodes production components. This sand is relatively fine and erodes production components in the flow path. In some completions, however, the well bore is uncased, and an open face is established across the oil or gas bearing zone. Such open bore hole arrangements are utilized, for example, in water wells, test wells and horizontal well completions. Similarly, after the well is completed and placed in production, formation sand from unconsolidated formations may also be swept into the flow path along with formation fluid.
With either cased or uncased well bores, one or more sand screens may be installed in the flow path between the production tubing and the perforated casing. A packer may be set above and below the sand screen to seal off the annulus in the producing zone from non-producing formations. The annulus around the screen may be packed with a relatively coarse sand or gravel which acts as a filter to reduce the amount of fine formation sand reaching the screen.
Conventionally, sand screens employ a perforated mandrel which is surrounded by longitudinally extending spacer bars, rods or ribs and over which a continuous wire is wrapped in a carefully spaced helical configuration to provide a predetermined longitudinal gap between the wire turns. See for example, U.S. Pat. No. 3,785,409; U.S. Pat. No. 3,958,634; and U.S. Pat. No. 3,908,256. The aperture between turns permits formation fluids to flow through the screen, while the closely spaced wire turns exclude fine particulate materials such as sand or gravel which may penetrate the gravel pack.
However, during the initial production period following the gravel packing operation, fine sand may be carried through the gravel pack before the gravel pack bridge stabilizes and yields clean production. Those fines tend to migrate through the gravel pack and screen and lodge within the inner annulus between the outer wire wrap and the perforated mandrel. In some instances, this can cause severe erosion of the screen and ultimate failure of the screen.
One attempt to overcome the sand erosion problem is to interpose a prepack of gravel within the annulus between the inner mandrel and the outer wire screen. The prepacked gravel is sized appropriately to exclude the fines which accompany the formation fluid. Raw gravel, as well as epoxy resin coated gravel, have been used extensively in prepacked well screens. However, the sand erosion problem has not entirely been alleviated, and erosion continues to remain a problem in some instances.